FIG. 1 is a block diagram of a mobile telephone 10 viewed from the perspective of its role as a receiver of information from the network with which it communicates. FIG. 1 shows only certain fundamental elements that are involved in the processing of signals that are received at the telephone 10. As shown in FIG. 1, the telephone 10 comprises an antenna 12, a quadrature downconverter 14, a bandpass filter 16, a demodulator 18 and an information sink 20. The purposes of these elements is well known and therefore will now be only briefly discussed.
Wireless signals acquired by the antenna 12 are supplied to the quadrature downconverter 14. The downconverter 14 shifts the acquired signals down in frequency from the RF (radio frequency) range to the IF (intermediate frequency) range. Hence, the acquired signals are said to be downconverted in frequency. In addition to downconverting the signals from the antenna 12 in frequency, the unit 14 also converts the acquired signals into a quadrature format.
The basic structure of the quadrature downconverter 14 is shown in FIG. 1. The signals that are received through the antenna 12 are supplied in parallel to mixers 22 and 24. The quadrature downconverter 14 also comprises a local oscillator 26, whose output is supplied to mixer 22 and, via 90° phase shifter 28, to mixer 24. The output of mixer 22 provides the in-phase component of the quadrature format downconverted signal 30 and the output of mixer 24 provides the quadrature phase component of the quadrature format downconverted signal.
The quadrature format downconverted signal 30 is then supplied to the bandpass filter (BPF) 16. The bandpass filtered quadrature signal, indicated 32, is then supplied to the demodulator 18. The demodulator 18 recovers an information signal 34 from the bandpass filtered quadrature signal 32 and supplies it to the information sink 20. The demodulator 18 can use various techniques to recover the information signal, as will be apparent to the skilled person. For example, the demodulator 18 could perform Viterbi equalisation on the bandpass filtered quadrature format signal 32. The sink could be, for example, a display screen or a speaker forming part of the telephone 10.
It will be apparent to the skilled person that the telephone 10 will comprise many other elements besides those shown in FIG. 1, for example an amplifier arranged to act on the signal from the antenna 12 before it reaches the quadrature downconverter 14 and an analog to digital converter to act on the bandpass filtered quadrature downconverted signal 32 before it is processed by the demodulator 18. However, these and other elements are not described in this document for the sakes of both brevity and clarity, the description instead concentrating on those elements that are most closely connected to the invention.
The frequency of the output of the local oscillator 26 can be varied to adjust the part of the spectrum of the signal acquired by antenna 12 that is downconverted to lie at the passband of the bandpass filter 16. However, the details of such channel selection schemes will be well known to readers skilled in this art.
Consider now the case where the signal acquired by the antenna 12 contains just a single active channel spanning a band of frequencies centred on an RF frequency f1. Mathematically, the spectrum of the signal acquired by the antenna 12 extending in the positive frequency domain can be regarded as reflected about 0 Hz to the negative frequency domain. FIG. 2 illustrates the spectrum of the signal acquired by the antenna 12 comprising the signal 33 in the active channel centred on frequency f1 and also its “reflection” 35, being a complex-conjugated version of the signal 12 but at frequency −f1.
The complex-conjugation is shown as an asterisk in FIG. 2 (and the same notation is used in those of the subsequent figures that illustrate spectra).
Consider also that the output of the local oscillator 26 is at frequency ω. The effect of the local oscillator signal on the frequency spectrum of FIG. 2 is to convert each component of that frequency spectrum into two components, one shifted down in frequency by ω (and hereinafter referred to as the “downshifted component”) and one shifted up in frequency by ω (and hereinafter referred to as the “upshifted component”). This is shown in FIG. 3. The quadrature downconverter 14 converts the signal 33 at f1 into a downshifted component 33a lying at f1−ω and an upshifted component 33b lying at f1+ω. Likewise, the quadrature downconverter 14 converts the signal at into a downshifted component 35a lying at −f1−ω and an upshifted component 35b lying at −f1+ω. Thus, each of the signals 33 and 35 at f1 and −f1 is converted into a pair of signals symmetrically disposed about the position of the original signal.
In each of these pairs, the lower frequency signal is regarded as the wanted signal and the other, unwanted, signal is regarded as an image signal (since it is symmetrically disposed beyond the original signal position). Accordingly, the quadrature downconverter 14 is designed to suppress these image signals and this suppression is apparent in FIG. 3 since the upshifted component of each pair is at a much lower power than the downshifted component of the pair. The difference in power of the two components in such a pair is a measure of the image rejection ratio (IRR) of the quadrature downconverter 14. However, in a practical downconverter, the IRR will never be perfect with the result that suppression of upshifted components will never be total. This imperfection in practical downconverters leads to certain problems as will now be discussed with reference to FIG. 4.
FIG. 4 pertains to the case where the spectrum of the signal acquired through antenna 12 contains a signal in a wanted channel, that is to be directed through the passband of the BPF 16, and a signal in a channel adjacent to the wanted channel and having significantly higher power than the wanted channel. FIG. 4 shows three power versus frequency spectra 36, 38 and 40. The frequency axes of these spectra are aligned with one another, for ease of comparison of their frequency content, and the passband of BPF 16 is also shown.
Spectrum 36 shows the spectrum of the signal acquired by the antenna 12. Again, the spectrum of the signal acquired by the antenna 12 can be considered mathematically as containing in the negative frequency region a “reflection” of what is contained in the positive frequency region. The signal in the wanted channel is indicated 42 and its negative frequency “reflection” is indicated 48. The higher power signal in the adjacent channel is indicated 44 and its negative frequency “reflection” is indicated 46.
Spectrum 38 shows, partially, the effect of the downconverter 14 on the positive frequency half of the spectrum 36. The wanted signal 42 is downconverted to yield a downshifted component 42a which lies in the passband of the BPF 16 whereas the higher power adjacent channel signal 44 is downconverted to yield a downshifted component 44a which lies just below the passband. Of course, the downconverter 14 also produces upshifted components for signal 42 and 44 but these components are not shown since they do not bear on the passband (and in any event would lie off the right hand side of the diagram).
Spectrum 40 shows, partially, the effect of the downconverter 14 on the negative frequency half of spectrum 36. The “reflections” 48 and 46 of the wanted and adjacent channel signals (respectively) are downconverted in frequency to yield respective downshifted components and upshifted components. The downshifted components do not bear on the passband and so are not shown (and in any event would lie off the left hand side of the diagram). The upshifted components, however, are shown. The upshifted component 46b of the “reflection” 46 of the adjacent channel signal appears in the passband and the upshifted component 48b of the “reflection” 48 of the wanted signal appears just below the passband. Of course, these upshifted components are suppressed in power to the extent possible given the design of the downconverter 14 (this extent is described by the downconverter's IRR).
It will be apparent that the downconverter 14 operates so as to place both the downconverted component 42a of the wanted signal 42 and the upconverted component 46b of the “reflection” 46 of the adjacent channel signal 44 in the passband of the bandpass filter 16. Accordingly, the upconverted component 46b can hamper the demodulation of the downconverted component 42a in the demodulator 18. It will also be appreciated that this problem will be worse the greater the power of the adjacent channel signal 44.